This invention relates to a method and apparatus to gauge the wall thickness of a tubular object by simultaneously measuring the thicknesses at several points on the periphery of a tubular object, in a non-contacting manner.
Generally, in manufacturing tubing such as seamless steel tubes, it is necessary to accurately gauge wall thicknesses or, inner and outer diameters, both in the case of a cold manufacturing process, where the tubing is at ambient temperature, and in the case of a hot manufacturing process, where the tubing has a temperature of about 1000.degree. C. The requirements for such a method of measurements are that it take place without contact of the tubing, that it be conducted under high temperature conditions of about 1000.degree. C., that the measurement have an accuracy of about 200 .mu.m.+-.50 .mu.for a wall thickness of about 5-40 mm, and that it be performed rapidly. A rapid process is necessary in order to detect frequent variations of the wall thickness which occur along the periphery and length of the tubing.
One method which has been heretofore proposed is illustrated in FIGS. 1 and 2. As seen in FIG. 1, the wall thickness of tubing 20 is measured along the parallel lines A, B, C, and D, where the line B contacts with the outer periphery of the tubing at a point a. Thus, the measured dimension L of wall thickness varies as shown in FIG. 2. Taken along the line A, which does not cross the tubing periphery, the dimension L is zero. Along the line B, which just contacts with the outer periphery of tubing 20, the dimension L is also zero. Along the line C, which contacts with the inner periphery of tubing 20, the dimension L reaches its maximum. Along the line D, where the dimension L is given by the sum of widths of both left and right wall portions, it is far smaller than the value taken along the line C. If the measurement takes place with the position (of the line) shifting in the direction of the arrow Y, the value of L follows the curve as shown in FIG. 2.
The wall thickness of tubing 20 is therefore given by a distance h in the direction of arrow Y between a point B of rising in front of that curve and a point C where the value of L reaches its maximum (FIG. 2). A measuring instrument comprising a radiation source and a radiation detector is used (not shown) to obtain this curve. The source and detector are placed on the line A, on both sides of the tubing, separated from each other by a distance greater than the diameter of tubing 20. This measuring instrument is moved in the direction of arrow Y, from a position on the line A through the positions of the lines B, C, and D and so on, while its output indicates the varying value of L. The distance of movement in the Y direction, between the point where its output just begins to rise and the point where it reaches the maximum, is the wall thickness.
The method just described can achieve the measurement without touching the measured object. However, it does not afford a high degree of accuracy in measurement, since an error in defining the position of the radiation beam causes an error in the measured value of wall thickness. It has another drawback in that rapid measurement is not readily achievable. Since a gamma-ray source is used for the radiation when measuring the thickness of a steel pipe wall a lengthy operation for measurement is required as the source is massive and cannot be quickly moved for radiation beam scanning.
In order to eliminate the drawbacks of this known measuring method, it is desired to create a new method for wall method thickness measurement which operates in a non-touching manner affording higher accuracy and rapid performance.